Ged 2237

version 01 07 December 2009
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AAUUSSTTEENNOOFFEERRRRIITTIICC
SSTTAAIINNLLEESSSS SSTTEEEELLSS
DDUUPPLLEEXX

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CCoonntteennttss
Contents --------------------------------------------------------------------------2
Foreword -------------------------------------------------------------------------3
Main applications ----------------------------------------------------------------4
Analyses and reference standards ---------------------------------------------6
1. Analyses-----------------------------------------------------------------------6
2. Reference standards ---------------------------------------------------------7
2.1. Normative equivalences ---------------------------------------------------------- 7
2.2. Associated normative documents------------------------------------------------ 7
Physical properties --------------------------------------------------------------8
Heat treatments and structure -------------------------------------------------9
Solution annealing------------------------------------------------------------- 10
Structural transformations --------------------------------------------------- 10
Mechanical properties --------------------------------------------------------- 12
Forging-------------------------------------------------------------------------- 17
Machining----------------------------------------------------------------------- 18
Welding ------------------------------------------------------------------------- 19
Corrosion resistance: examples of the use of duplex steels--------------- 20
1. Introduction to the corrosion resistance of duplex grades ------------ 20
2. Use of duplex steels in the chemical and paper manufacturing
industries ----------------------------------------------------------------------- 22
3. Use of duplex steels in the building industry: for example concrete
reinforcing bars ---------------------------------------------------------------- 29
4. Use of duplex steels in the petrochemical industry: stress corrosion
problems. ----------------------------------------------------------------------- 30
Additional information -------------------------------------------------------- 32

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FFoorreewwoorrdd
Stainless steels with a high content of chromium and molybdenum, the major
elements in corrosion resistance, are often recommended for use in certain very
aggressive environments.
For several decades, the market share of austenoferritic or "duplex" stainless steels
has been increasing.
Although, for many years, the use of duplex steels was almost exclusively restricted
to the production of components which were cast and then forged, they are now
available in an extensive range of long or flat laminated products.
Their outstanding characteristics, combining high mechanical properties with often
exceptional corrosion resistance and their low cost – together with their low nickel
content – make them attractive to industries that traditionally use high alloy grades:
Cellulose and paper pulp industry;
Oil industry;
Waste and effluent treatment;
Phosphoric and sulphuric derivative mineral chemical industry;
Building industries (see the special technical documentation), etc.
The purpose of this technical documentation is to help users to choose the
right Duplex grades by giving them advice on how to proceed.
In order to be as comprehensive as possible, every effort has been made
to compare these products with well-known reference stainless
steels:
4404 (316L) and its improved machinability version UGIMA®
.
4539 (904L), the "superaustenitic" grade, which is the reference grade for
highly chlorinated environments (brine, sea-water treatment) where
the risks of localised pitting or crevice corrosion are considerable.

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MMaaiinn aapppplliiccaattiioonnss
Although the majority of duplex steels are well known and widely used in the
chemical and oil industries, their use is quickly spreading to the "green" industries
associated with water and soil pollution control or waste processing.
Increasingly strict sanitary measures are being applied in the agri-food and health
industry at the same time as stricter controls are being introduced into the chemical
sanitisation environments: several occurrences of corrosion encountered on
conventional grades (1.4307, 1.4404) can only be resolved by changing to "nobler",
more appropriate solutions, such as Duplex grades.
Where long stainless steel products are concerned, such solutions will be of
particular interest for fittings or mechanical components used in welded sheet metal
assemblies, fluid systems or structures in the following fields:
Bolts and screws Cables and tie rods
Filters Handling hooks
Chains Mixers, blenders
Probe supports Various mechanical components
Valves and fittings Connections and flanges
Pump shafts
Building and civil works
Rams
Reinforcements, anchor bars
Duplex stainless steels are particularly recommended for use in the industries and
applications listed below, although this list is by no means exhaustive:
PVC and chlorinated polymer synthesis;
Phosphoric acid and by-products (fertilizers, explosives);
Sulphuric acid and by-products;
Cellulose and paper pulp processing;
Textile fibre bleaching;
Boring and extraction;
Off-shore;
Refining;
Tidal power plant equipment;
Sea-water nuclear power stations;
Off-shore wind turbines;
Soft water production by desalination;
Chemical
Oil
Energy
Sea water

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Thalassotherapy equipment;
Fish farming;
Underwater work;
Nautical equipment;
Anchor bars;
Concrete reinforcement bars; (see the special documentation)
Dialysis equipment;
Thermalism;
Sanitisation and sterilisation;
Water treatment;
Waste and effluent treatment;
Brines (cheeses and cooked meats);
Mustard and vinegar;
Wine (sulphite treatment);
Building - civil
works
Health
Environment
Agri-food

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AAnnaallyysseess aanndd rreeffeerreennccee ssttaannddaarrddss
1. Analyses
An analysis of Duplex grades is given in Table1; later in the document, the
properties will be compared with those of the standard UGI 4404 (316L) or UGIMA®
4404 grades (improved machinability variant, considered to be the minimum
requirement for harsh corrosive environments) and, at the top end of the scale, with
those of UGI 4539 (904L), the superaustenitic grade.
Table1: Analyses
Grade C Si Mn Ni Cr Mo S P Cu N
UGI 4404
UGIMA® 4404
 0.03  1  2
10
11
16.5
17.5
2
2.5
0.015
0.030
 0.040 - -
UGIMA® 4460  0.03  0.75  1
4.5
5
26
27
1.3
1.8
0.005
0.025
 0.035 -
0.05
0.2
UGI 4362
(UGI 35N)
 0.03  1.0  2
3.5
5.5
22
24
0.1
0.6
 0.015  0.035
0.1
0.6
0.05
0.2
UGI 4462
(UGI 45N)
 0.03  0.75
1
2
5
6
22
23
2.5
3.5
 0.01  0.035 -
0.11
0.22
UGI 4507
(UGI 52N+)
 0.03  0.7  1.5
6
7
24.5
26
3.3
4
 0.01  0.035
1.2
2
0.15
0.30
UGI 4539
(UGI 904L)
 0.03  1  2
24
25
19
20
4
5
 0.01  0.025
1.2
2
 0.15

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2. Reference standards
2.1. Normative equivalences
UGI
EN 10088
Numerical
EN 10088
Alphanumeric
AISI
UNS
and others
UGI / UGIMA® 4404 1.4404 X2 CrNiMo 17-12-2 316L UNS S31603
UGIMA® 4460 1.4460 X3 CrNiMoN 27-5-2 329
SUS 329J1
SIS 2324
UGI 4362 (35N) 1.4362 X2 CrNi 23-04 - UNS S32304
UGI 4462 (45N) 1.4462 X2 CrNiMoN 22-5-3
ASTM A
182-F51
UNS S31803
UNS S32205
SIS 2377
SUS 329532
UGI 4507 (52N+) 1.4507 X2 CrNiMoCuN 25-6-3
ASTM A
479
UNS S32550
SUS 39542
UGI 4539 (904L) 1.4539 X1 NiCrMoCu 25-20-5 904L UNS N08904
2.2. Associated normative documents
EN 10088-1 Stainless steels – List of stainless steels
EN 10088-3 Stainless steels – Semi-finished products, bars, wire
rods, cold-drawn wires, profiles and cold-finished
profiles in corrosion resistant steel for building and
general use.
EN 10272 Stainless steel bars for pressure vessels
ASTM A276 Stainless and heat-resisting bars/shapes
ASTM A479 / ASME SA 479 Stainless steel bars for boilers and other pressure
vessels
NACE MR0175
NF XP A 35-014
Sulphide stress cracking resistant material for oil
field equipment
Steels for reinforced concrete: smooth stainless
steel lock or print bars and coils

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PPhhyyssiiccaall pprrooppeerrttiieess
Table 2: Physical properties
Symbol Unit Temperature
Value
4404 4460 4362 4462 4507 4539
Density d
No
dimensions
4°C 7.9 7.9 7.8 7.8 7.9 8.05
Specific heat c J.kg.°C 20°C 500 500 490 400 500 500
Thermal
conductivity
k W/m.°C 20°C 15 15 17 16 17 14
Linear
expansion ratio
 10-6
m/m.°C
20 to 100°C
20 to 300°C
19
20
13
13.5
13
14
13
14
12.5
13.5
15.1
16.8
Electrical
resistivity
 µ.cm 20°C 76 80 80 70 80 80
Longitudinal
elasticity
module
E MPa.103
20°C 200 200 200 200 205 205
Poisson
coefficient

No
dimensions
20°C 0.30 0.30 0.30 0.30 0.28 0.28
A comparison of physical properties indicates the lower expansion ratio and higher
thermal conductivity of Duplex steels.
Figure 1: Comparison of the thermal conductivity of austenitic stainless steels and
duplex steels (comparison of average values)
10
12
14
16
18
20
22
24
0 100 200 300 400 500 600
Température °C
ConductivtéthermiqueenW/m.°C
Austénitique DuplexAustenitic
Temperature
ThermalconductivityinW/m.C°

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HHeeaatt ttrreeaattmmeennttss aanndd ssttrruuccttuurree
All the grades referenced are used in the solution-annealed state under the
conditions described in Figure 2a.
Figure 2a: Solution annealing values according to grade
940
960
980
1000
1020
1040
1060
1080
1100
1120
1140
1160
1180
Temperature(°C)
T°C min 1025 1030 950 1030 1040 1075
T°C max 1100 1100 1050 1100 1120 1150
Ugine 4404 Ugima 4460 Ugine 4362 Ugine 4462 Ugine 4507 Ugine 4539
Solution
annealing

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SSoolluuttiioonn aannnneeaalliinngg
The structure of Duplex steels after annealing is two-phase ferrite + austenite, with
a percentage of ferrite - appropriate for the optimisation of mechanical properties
and corrosion resistance - of between 40 and 70%, depending on the grades.
The respective percentages of austenite and ferrite can vary according to the
percentage of hot working and the temperature of the heat treatment.
Figure 3: Structures of austenitic and Duplex steels
Austenitic steel Duplex steel
Structural transformations
Compared with standard austenitic steels (1.4307, 1.4404), Duplex grades are liable
to undergo various types of structural transformations depending on the
temperature.
 phase precipitation occurs when the steel is kept within a temperature range of
600 - 900°C. It causes embrittlement at ambient temperature and must therefore
be avoided.
' phase precipitation can occur after the steel is kept at a temperature of between
350 and 550°C for a prolonged period. This embrittling phase weakens the
resilience and reduces corrosion resistance.
 phase
' phrase

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Figure 4: ' and  phase precipitation TTT curves in Duplex steels
The  and ' phases can easily be avoided if the thermal cycles (forging,
for example) are sufficiently controlled. The limit temperature at
which duplex steels should be used is 300°C.
UGI 4362
UGI 4507
UGI 4462 Intergranular precipitates
Ferrite
sigma
phase
chi
phase
sigma
phase
’
phase
’
phase
Time (h)
core
skin
sigma phase
carbides
’ phase

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MMeecchhaanniiccaall pprrooppeerrttiieess
A comparison of the mechanical properties between the different families of
stainless steels – with the exception of martensitic steels whose behaviour is more
similar to that of alloy steels - reveals that the best compromise between tensile
strength and resilience is obtained with duplex steels.
Table 3 compares the typical mechanical properties for the different families of
stainless steels (with the exception of martensitic stainless steels).
Table 3: Comparison of the mechanical properties of stainless steels
Type of steel Rm (Mpa) Rp0.2 (Mpa) KV (in J)
Ferritic 450 to 600 280 to 360 10 to 20
Austenitic 550 to 700  250  200
Duplex 650 to 750  480  150
The values shown in Table 4 refer to the annealed condition.
Table 4: Mechanical properties at ambient temperature
Grade Rm (Mpa)
Rp0.2 (Mpa)
min
A % min KV (J) min
UGIMA® 4404 460 – 660 185 40 150
UGIMA® 4460 620 – 880 450 20 85
UGI 4362 (35N) 600 - 830 400 25 100
UGI 4462 (45N) 660 - 860 450 25 100
UGI 4507 (52N+) 690 - 890 490 20 100
UGI 4539 (904L) 530 - 730 230 35 100
Once again, we strongly advise against "hardening" Duplex steels.
Mechanical
properties at
ambient
temperature

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The figures below illustrate the variations in Rm and Rp0.2 versus grades:
Figure 5: Resistance values Rm for various grades
400
500
600
700
800
900
1000
RmMPa
Rm Min MPa 460 620 600 660 690 530
Rm Max MPa 660 880 830 860 890 730
Ugine 4404 Ugima 4460 Ugine 4362 Ugine 4462 Ugine 4507 Ugine 4539
Figure 6: Yield strength value Rp0.2 for various grades
Ugine 4362
Ugine 4462
Ugine 4507
Ugine 4539
Ugima 4460
Ugima 4404
100
150
200
250
300
350
400
450
500
550
600
=
Rp0.2 Min 185 450 400 450 490 230
Ugima 4404 Ugima 4460 Ugine 4362 Ugine 4462 Ugine 4507 Ugine 4539
Rp0.2-MPa

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In view of the structural transformations that are liable to alter their working
properties, the use of Duplex grades above 300°C is not recommended.
Table 5: Mechanical properties at high temperature (minimum Rm and Rp0.2 values)
UGI and UGIMA® 4404
Test
temperature
100°C 150°C 200°C 250°C 300°C 350°C
Rm MPa min 430 410 390 385 380 380
Rp0.2 MPa min 165 150 137 127 119 113
UGIMA® 4460
Test
temperature
100°C 150°C 200°C 250°C 300°C 350°C
Rm MPa min 610 580 565 550 - -
Rp0.2 MPa min 360 335 310 295 - -
UGI 4362 (UGI 35N)
Test
temperature
100°C 150°C 200°C 250°C 300°C 350°C
Rm MPa min 570 - 530 - 490 -
Rp0.2 MPa min 330 - 280 - 230 -
UGI 4462 (UGI 45N)
Test
temperature
100°C 150°C 200°C 250°C 300°C 350°C
Rm MPa min 620 595 580 580 - -
Rp0.2 MPa min 360 340 320 310 - -
UGI 4507 (UGI 52N+)
Test
temperature
100°C 150°C 200°C 250°C 300°C 350°C
Rm MPa min 680 655 640 640 - -
Rp0.2 MPa min 400 380 360 350 - -
UGI 4539 (UGI 904L)
Test
temperature
100°C 150°C 200°C 250°C 300°C 350°C
Rm MPa min 500 480 460 450 440 440
Rp0.2 MPa min 205 190 175 160 145 140
The superaustenitic grade UGI 4539 (Ugine 904L) can be used up to about 600°C,
but is not particularly useful from the technical point of view, nor is it at all
economical compared with more conventional "refractory" grades at this
temperature range.
The figures below illustrate the variation in tensile strength versus temperature for
the different grades:
Mechanical
properties at
high
temperature

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Figure 6: Tensile strength versus temperature
300
350
400
450
500
550
600
650
700
100°C 150°C 200°C 250°C 300°C 350°C
Température °C
RmMpa
Ugima 4404
Ugine 4539
Ugima 4460
Ugine 4462
Ugine 4507
Ugine 4362
Figure 7: Yield strength versus temperature
100
150
200
250
300
350
400
450
500
100°C 150°C 200°C 250°C 300°C 350°C
Température °C
Rp0.2Mpa
Ugima 4404
Ugine 4539
Ugima 4460
Ugine 4462
Ugine 4507
Ugine 4362
The "cryogenic" applications are not, strictly speaking, applications where corrosion
is a serious problem (e.g.: transportation of liquified liquids); in fact, below a certain
temperature threshold, the development kinetics of most forms of corrosion are
much slower and the majority of "standard" austenitic grades (UGIMA® 4307,
UGIMA® 4404) are perfectly suitable for most applications.
In some specific cases, if grades that can be used for a wide range of operating
temperatures (for example + 100 to - 100°C) are required for use in highly
corrosive environments, the choice of grade will be decided by the level of
mechanical properties required for the components.
In the case of high stress at ambient temperature and ductility requirements at
below -60°C, only a structurally hardened grade such as UGI 4944 (e.g. AFNOR
Mechanical
properties at
low
temperature
Temperature °C
Temperature °C

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Z6NCTDV 25-15 B – ASTM grade 660) is an appropriate solution for a highly
corrosive environment.
Table 6: Preferred parameters for cryogenic applications
Operating conditions Possible choice
Moderate tensile strength requirements
(however, the tensile strength increases on
austenitic and Duplex steels when the
temperature decreases);
Operating temperature < -60°C;
Highly corrosive environment at ambient
temperature.
UGIMA® 4404 possible, many corrosive
environments become slightly aggressive at
low temperature;
In the event of heat variations, where the
top temperature may exceed 15 to 20°C, UGI
grade 4539 should be used to ensure
maximum safety.
Operating temperature above - 60°C;
Stricter requirements for tensile strength;
Highly corrosive environment at ambient
temperature.
Duplex grade to be selected – according to
the corrosive environment – from those
recommended (their resilience transition
temperature is in the region of –60°C)
Figure 8: Transition curves showing the resilience of UGI grades 4462 and 4507
0
50
100
150
200
250
300
Température °C
KV(Joules)
Ugine 4462 260 250 225 160 100 48
Ugine 4507 275 260 250 175 100 40
50 0 -20 -50 -75 -100
Température de transition -60°C
Temperature °C
Transition temperature -60 °C

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FFoorrggiinngg
The forgeability of these steels is acceptable between 1200 and 900°C, which is,
however, less than that of current austenitic steels (1.4307, 1.4404).
Their deformability at high temperatures depends, for a given temperature, on the
ferrite content, as a high ferrite content improves forgeability.
The forgeability of 1.4462 and 1.4507 grade steels is slightly lower below 1100°C,
due to their nitrogen content.
Table 7: Duplex steel forging conditions
Preheating Forging Cooling
 Direct oven loading at the
forging temperature for small
components (~ 1200°C);
 When forging large
components, it can prove useful
to preheat them at a
temperature slightly above
850°C to ensure that their
structure is properly
homogenised.
 Between 1200 and 900°C
 A reduction in forgeability may
occur below 1100°C on grades
with a high nitrogen content
(1.4507)
 As quickly as possible below
900°C to prevent  phase
formation
Duplex steels

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MMaacchhiinniinngg
The corrosion resistance requirements of these grades considerably limit the
possibilities of resulphurisation with a view to improving the machinability
properties.
Although the S content "authorised" by the standards is less than 0.010%, in
practice it only rarely exceeds 0.005%. Under such conditions, it is often very
difficult to machine these grades.
The two-phase structure of these steels where each phase performs
differently during machining makes them more difficult to machine than austenitic
stainless steels. They put a great strain on the tools (risk of vibrations, coating
chipping) if they are not machined under optimum cutting conditions and if the tools
used are not of the correct quality. Furthermore, they require the use of coated
carbide inserts and low cutting speeds, as opposed to austenitic stainless steels.
Fig 9: Machinability of duplex grades
0
50
100
Débit copeaux
(cm3/mn)
Tournage outil carbure revêtu
4404
4460
4462
0
1
2
3
Débit copeaux
(cm3/mn)
perçage acier rapide
4404
4460
4462
Austenoferritic
steels
Chip rate
(cm3/mn
Coated carbide toll turning
Chip rate
(cm3/mn
High-speed steel drilling

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WWeellddiinngg
The percentage of ferrite in the molten areas of these grades is higher when the
composition of the filler metal is identical to that of the base metal. This should be taken
into account when the optimum composition of the filler metal is determined. In addition,
the areas affected by the heat are also liable to contain more ferrite than the base metal. To
minimize this difference, high linear energy welding is recommended to reduce cooling
times. However, only energy that does not cause  phase formation should be used:
- for UGI 4362, there is very little risk
- for UGI 4462, the energy must be less than 2kJ/cm
- for UGI 4507, see the table below:
BUTT WELDING CORNER WELDING
Process Pulsed MIG TIG Pulsed MIG TIG
Gas Ar 95.5% + CO2 1.5% + N2 3% Ar + N2 4%
Ar 95.5% + CO2
1.5% + N2 3%
Ar + N2 4%
Sheet
metal
thickness
(mm)
Min. weld
energy
(KJ / mm)
Max. weld energy
(KJ / mm)
Min.
weld
energy
(KJ /
mm)
Max.
weld
energy
(KJ /
mm)
Min.
weld
energy
(KJ /
mm)
Max.
weld
energy
(KJ /
mm)
Min.
weld
energy
(KJ /
mm)
Max.
weld
energy
(KJ /
mm)
4.76 0.38 0.47 0.60 0.80 0.60 0.77 1.00 1.30
6.35 0.55 0.65 0.90 1.10 0.73 1.05 1.24 1.73
7.93 0.65 0.87 1.10 1.45 0.80 1.22 1.60 2.05
9.50 0.73 1.05 1.24 1.75 0.85 1.30 1.60 2.15
12.00 0.94 1.15 1.60 1.95 0.97 1.35 1.60 2.20
16.00 0.95 1.30 1.60 2.20 0.97 1.35 1.60 2.20
19.00 0.97 1.32 1.60 2.20 0.97 1.35 1.60 2.20
26.00 0.97 1.35 1.60 2.20 0.97 1.35 1.60 2.20
A linear energy welding area where the two above-mentioned risks are minimised can
therefore be determined. The thicker the components to be welded, the higher the energy in
this area (i.e. rapid weld cooling).
It is not advisable to preheat the components prior to welding.
Components should not be heat treated after welding, but the solution annealing described
in the "Structure heat treatment" section may be performed, if necessary.
The filler metals contain a higher nitrogen and/or Ni content than the base metal in order to
optimise the ferrite content in the molten areas, see the table below:
Duplex grades
Filler wire 
Base metal 
ER2209
22.9.3NL
SMArc 45N
ER2553
Z3CND25-06-03Az
SMArc 52N
ER 309Lsi
23.12Lsi
SMArc 309LM
UGI 4362  
UGI/UGIMA® 4460 
UGI4462 
UGI 4507 

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CCoorrrroossiioonn rreessiissttaannccee:: eexxaammpplleess ooff tthhee uussee ooff
dduupplleexx sstteeeellss
1. Introduction to the corrosion resistance of duplex grades
Laboratory tests such as the "accelerated corrosion electrochemical test" have been
widely used to study the corrosion of duplex or austenoferritic grades. A great deal
has been written on this subject and we will simply mention grade classifications in
a few laboratory electrochemical tests.
Figure 10 below compares 1.4462 / 45N duplex with standard austenitic grades
304L 1.4306 and 316L 1.4404 in an accelerated fatigue - corrosion test, which
was performed in different environments: in air and for environments with pH
values ranging from neutral to highly acid.
In all these cases, the duplex steel performed best.
Figure 10: Comparison of duplex 4462 with two standard austenitic stainless steels
Figure 11 shows the critical pitting temperature versus the mechanical strength for
two duplex steels (45N / 1.4462 and 52N / 1.4507) in comparison with two austenitic steels
(316L / 1.4404 & 904L / 1.4539).
This critical pitting temperature is determined according to the accelerated corrosion test
(ASTM G48 Standard) in a 6% ferric chloride environment.
0
50
100
150
200
250
300
350
400
450
500
1.4306
(304L)
1.4404
(316L)
1.4462/ 45N
(Duplex)
Limite de fatigue (MPa) à 20E+7 cycles
Air
pH 7
pH 3
pH 1
Fatigue limit (MPa) at 20E+7 cycles

- 21 -
Figure 11: Critical pitting temperature versus mechanical properties
Note the excellent behaviour of 4507 duplex with respect to its mechanical
properties and pitting corrosion resistance.
Note: Example of UGIMA®
4460
As seen above, this grade is particularly useful, due to its machinability.
The following table compares the performances of UGI 4404 / 316L and UGIMA®
4460 / 329 stainless steels (for a more detailed comparison, sulphur contents of
0.02% were chosen).
Machinability
Overall
general
corrosion in
an H2SO4
acid
environment
Stress
corrosion
Localised
crevice
corrosion
Localised
pitting
corrosion
UGIMA®
4460
329
Reference "BASE 100"
Much less
sensitive
"BASE 100" Reference
UGI 4404
316L
Reference
Much less
effective
(by 750%)
Sensitivity as
for all
austenitic
grades
Slightly less
effective
(by
approximately
10%)
Reference
Ugine 4507
Ugine 4462
Ugine 4539
Ugine 4404
0
10
20
30
40
50
60
70
80
0 100 200 300 400 500 600
Rp0,2 mini (MPa)
T°piqûrePittingT°
Rp0.2 min (Mpa)

- 22 -
2. Use of duplex steels in the chemical and paper manufacturing
industries
The overall general corrosion is marked by an even dissolution of the metal in
contact with the corrosive environment. The results of this type of corrosion can be
quantified in terms of loss of weight or thickness (mm/year, for example); a grade
use limit of 0.2 mm / year is often acceptable.
The overall general corrosion on stainless steels is, for example, found in "strong"
acids (sulphuric acid, phosphoric acid) and is a type of corrosion often encountered
in the chemical industry; localised corrosion conditions may also be encountered
(pitting, crevice, intergranular or stress corrosion), which is naturally far more
difficult to quantify and anticipate).
RRREEECCCOOOMMMMMMEEENNNDDDAAATTTIIIOOONNNSSS FFFOOORRR UUUSSSEEE IIINNN PPPHHHOOOSSSPPPHHHOOORRRIIICCC AAACCCIIIDDD
Phosphoric acid is a non-oxidising mineral acid. Industrial phosphoric acid is a very complex
acid in terms of corrosion: it contains many impurities which can adversely affect corrosion
resistance (hydrochloric acid; hydrofluoric acid; sulphuric acid), but it can also sometimes
contain certain foreign bodies that have a beneficial effect on the corrosion resistance of the
material in question (ferric and aluminium ions).
Recommendations for materials for the main stages in industrial phosphoric acid
manufacturing:
ENVIRONMENTS
and STAGES
MAIN
PROBLEM
Recommendations
for a pure
environment
AGGRAVATING
FACTORS
UGITECH
RECOMMENDATIONS
Pure and aerated
phosphoric acid
Uniform surface
attack
* Temperature <
90°C
UGIMA®4404
* Temperature
90-200°C
UGI4539
Natural
phosphate attack
stage
(see Biblio 1 for
further
information
about this stage)
General attack
related to
depassivation due
to fluorinated ions,
excessive amounts
of H2SO4, etc.
Grade 4404 not
recommended
UGI4539 Considerable
abrasion
phenomena
UGI 4507
(Behaves better than
4539 in attack tanks
- stirrers with respect
to corrosion–
abrasion
phenomena).
Slurry filtration
stage
(see Biblio 2)
UGIMA 4404 or
UGI 4362 with no
chlorides
Presence of
chlorides
UGI 4539 up to
2000 ppm of
chlorides.
 UGI 4507 up to
3000 ppm of
chlorides.
Phosphoric acid

- 23 -
RRREEECCCOOOMMMMMMEEENNNDDDAAATTTIIIOOONNNSSS FFFOOORRR UUUSSSEEE IIINNN SSSUUULLLPPPHHHUUURRRIIICCC AAACCCIIIDDD
Sulphuric acid is a non-oxidising mineral acid; it acts as a reducing agent if
its concentration is less than 50% and as an oxidising agent if its concentration
exceeds 80% (these limits vary according to the temperature). Its degree of
ionisation is highest for a concentration of 30%, which leads to an irregularity in
fields where grades with a maximum acid aggressivity of between 40 and 80% are
used. The solubility of the oxygen is minimum at a concentration in the region of
70%.
ENVIRONMENTS MAIN
PROBLEM
Recommendations
for a pure
environment
AGGRAVATING
FACTORS
UGITECH
RECOMMENDATIONS
Pure & aerated
sulphuric acid
(see Biblio 3)
Uniform
surface
attack
*Concentration 40 to
85% UGI4539
(but for
temperatures not
exceeding 35°C at
high concentrations)
* Concentration <
40% or > 85%
UGI4507
Chlorides in
industrial acids
(On the one
hand, these ions
act as a reducing
agent and, on
the other, they
disturb passivity
because they are
adsorbed in
place of
hydroxide ions).
 UGI 4539
(highly recommended
with high percentages
of chlorides up to 2000
ppm)
H2SO4 with
oxidizing
impurities
(ferric ions,
etc.)
(see Biblio 4)
Uniform
surface
attack
Factors that can have a
beneficial effect on
grade behaviour.
This iso-corrosion diagram was produced for pure acid: it is therefore limited with respect to the use of grades for
low concentrations (<40%) compared with the diagram that might be produced if oxidising impurities were present.
The operating limits are given for a maximum corrosion rate of 0.2 mm/year. They obviously apply to the use of
stainless steel grades for welding, forming, etc. according to best practice.
Sulphuric acid
Note:
The 4362 grade can be
used instead of 4404 if
the H2SO4 content is
less than 13%

- 24 -
RECOMMENDATIONS FOR USE IN HYDROCHLORIC ACID
Hydrochloric acid is a mineral acid reducing agent.
The problems mainly involve general corrosion due to very high acid activity and the
fact that this reducing medium has an adverse effect on the passivation of stainless
steel.
The presence of oxidising agents therefore increases the dissolution rate of the
steel.
The aggressive nature of hydrochloric solutions may therefore vary greatly for a
given concentration, depending on whether or not the environment is in contact
with the air, or whether or not it contains impurities that can be reduced.
Consequently, high carbon grades should not be used (this is not specific to
hydrochloric acid, as dechromisation should also be avoided for many other
environments) and steels with high chromium and molybdenum contents, which are
required for concentrated solutions must be chosen very carefully, taking their
structural stability into account.
There may, of course, also be a risk of localised pitting corrosion in certain cases,
but only with the most corrosion resistant grades for which the overall general
corrosion is low.
When the passive film is unable to form, i.e. in all cases except in highly diluted
environments, the chromium no longer has a beneficial effect, since it dissolves in
the solution; the beneficial elements are therefore nickel (reduction in H2
overpotential) and molybdenum, which is stable (does not dissolve).
ENVIRONMENTS
MAIN
PROBLEM
Recommendation
for a pure
environment
AGGRAVATING
FACTORS
UGITECH
RECOMMENDATIONS
Pure and de-
aerated
hydrochloric
acid
General
attack on
the surface,
as the
environment
is
detrimental
to
passivation
Grade
4307
should not
be used.
* Temperature <
60°C and
concentration < 2%
UGI4507
* Temperature <
20°C and
concentration < 3%
UGI4539
UGI4507
Presence of
oxidizing agents
(chlorine or iron
chlorides) or
aeration
(dissolved oxygen,
etc.)
625 or 2.4856
For transfer or storage
facilities at ambient
temperature.
Temperature < 20°C
and concentration <
2%
or
Temperature < 50°C
and concentration <
1%
UGIMA®4404
UGI4362
High temperature
 risk of stress
corrosion
 UGI 4507
 UGI 4539
In certain processes where
condensates enriched with
small quantities of HCl are
formed and cracks occur in
grades 4307 and 4404,
904L or 52N+ may prove to
be corrosion resistant (PVC
manufacturing, for
example)
Hydrochloric
acid

- 25 -
RECOMMENDATIONS FOR USE IN A BASE such as sodium hydroxide NaOH
or potash KOH
The soda and potash solutions are only slightly corrosive for stainless steels,
whatever the concentrations, when the temperature does not exceed 100°C.
ENVIRONMENTS
MAIN
PROBLEM
Recommendation
for a pure
environment
AGGRAVATING
FACTORS
UGITECH
RECOMMEND-
ATIONS
Basic environment
Sodium carbonate
or pure potash
General
surface
attack
For temperatures <
90°C
UGIMA®4307
(see BIBLIO 5 for
more information)
High temperature >
100°C  risk of
stress corrosion and
general corrosion.
Up to 120°C
UGI 4539
UGI 4507
UGI 4462
Industrial sodium
carbonate or
potash
Grade
304L not
recomm-
ended
Presence of
chlorides and
chlorates  risk of
stress corrosion
Up to 80°C:
 UGI 4362
Up to 100°C:
UGI 4462
Up to 140°C:
UGI 4507
Industrial soda solutions are produced by sodium chloride electrolysis and are
polluted by chlorides and chlorates whose concentrations vary from one
manufacturing unit to another and also according to the soda concentration.
Typical solution: 50% NaOH; 1 to 5% NaCl and 0.1 to 1% NaClO3.
For low chlorate contents, there is little effect on the uniform corrosion of materials,
even at 150°C.
However, the risk of stress corrosion cracking is considerably increased by the
presence of such pollution. This type of corrosion is highly complex, and a
transgranular cracking mechanism with high concentrations of chlorides and
chlorates is observed in these basic environments, as in neutral chlorinated or
acidified environments.
It must be emphasised that the limit temperature at which cracking appears can
vary considerably, depending on the level of local stress on the material, as well as
on the aeration and concentration of the manufacturing environment.
RREECCOOMMMMEENNDDAATTIIOONNSS FFOORR UUSSEE IINN TTHHEE PPAAPPEERR MMAANNUUFFAACCTTUURRIINNGG
IINNDDUUSSTTRRYY::
In this case, sulphides are a corrosion aggravating factor. The processes used to
manufacture cellulose from wood (KRAFT process) involve attacking the wood chips
at 170°C, under pressure, with a liquor composed of 20% soda, to which sodium
sulphide Na2S, sodium carbonate Na2CO3 and traces of sodium thiosulphate
Na2S2O3 have been added.
During curing cycles at a temperature between 70°C and 170°C, there is a change
in the chemical composition of the environment, which then contains organic
impurities in addition to polysulphides.
Grades with added Molybdenum and Nickel are not recommended for these
environments; nickel, in particular, has an adverse effect in the presence of sulphur
compounds, as it forms complexes.
Soda and potash
Paper
manufacturing
industry

- 26 -
Selecting materials for the paper pulp industry:
CHIP PREPARATION (ABRASION PROBLEMS)
Digester; conveyors and storage; crusher; screens; defibrators
 UGI 4462 and UGI 4362
DELIGNIFICATION USING THE KRAFT PROCESS
 Vapour (localised corrosion + stress corrosion)
 UGI 4462; UGI 4507
 Preheating; reactor (overall general corrosion + stress corrosion)
 UGI 4462; UGI 4507 and UGI 4539.
 Impregnation; reactor (localised corrosion + stress corrosion + corrosion – abrasion)
 UGI 4462 and UGI 4362
 Storage of black and green liquor (overall general corrosion)
 UGI 4362 and UGI 4404
BLEACHING
 Washing and filtration (pitting corrosion)
 UGI 4462; UGI 4362; UGI 4301 and UGI 4404
 High density storage and reactor (pitting corrosion)
 UGI 4462 and UGI 4404
 Washing and filtration (pitting corrosion)
 UGI 4462; UGI 4362; UGI 4301 and UGI 4404
 Chlorine bleaching: tower (very severe pitting corrosion) Ti;
diffuser or washer and filtration tanks  UGI 4507
 Bleaching / sodium carbonate treatment (pitting corrosion)
 UGI 4462, UGI 4507 and UGI 4404,317L
 Bleaching / hypochlorite (pitting corrosion); tower
 UGI 4539
washing and filtration (pitting corrosion)
 UGI 4539; UGI 4462; 317L; UGI 4507
 Hydrogen peroxide bleaching: tower; washing and filtration (pitting corrosion)
 UGI 4362; UGI 4462; 317L; UGI 4507
PAPERMAKING
 High density storage (corrosion)
 UGI 4362 and UGI 4404
 Pulps and hydropulps (fatigue corrosion / abrasion corrosion)
 UGI 4362 UGI 4462, 317L
 Drive head (localised corrosion)
 UGI 4462, UGI 4507
 Cylinders (fatigue corrosion / stress corrosion)
 UGI 4507, UGI 4462
 Pneumatic conveyor (corrosion abrasion)
 UGI 4462, UGI 4362

- 27 -
RRREEECCCOOOMMMMMMEEENNNDDDAAATTTIIIOOONNNSSS FFFOOORRR UUUSSSEEE IIINNN AAACCCEEETTTIIICCC AAACCCIIIDDD
The acidity in an aqueous solution rapidly increases in line with an increase in the
concentration, making this product relatively aggressive.
ENVIRONMENTS
MAIN
PROBLEM
Recommendation
for a pure
environment
AGGRAVATING
FACTORS
UGITECH
RECOMMENDATIONS
Pure
acetic acid
General
surface
attack
* For temperatures
< 80°C, and for all
concentrations.
UGIMA®4307
At 120°C, UGIMA®
4307 can be used up
to concentrations of
20%.
* In boiling condition
and for a
concentration of 50%:
UGI4362
* In boiling condition
and for all
concentrations:
UGI4539
UGI4507
UGI4462
Acetaldehyde
oxidation process
Presence of acetic
anhydride impurities
in the separation
column and in
particular at the
bottom (at 150°C)
Presence of
byproducts at 200°C
of the hydrocarbon
chain oxidation
reaction
Grade 4404 not
recommended
* bottom part of the
column at 150°C: stainless
steel not recommended.
* median part of the
separation column:
 UGI 4539
* top part of the column
 UGIMA® 4404
or
 UGI 4362
* for the hydrocarbon
chain oxidation process in
a liquid environment
 UGI 4539
Acid transport and
storage
(exchangers,
heating coils)
UGIMA®4404
or
UGI4362
The operating limits are given for a maximum corrosion rate of 0.1 mm/year. They obviously apply to the use of
stainless steel grades for welding, forming, etc. according to best practice.
Acetic acid

- 28 -
RREECCOOMMMMEENNDDAATTIIOONNSS FFOORR UUSSEE IINN FFOORRMMIICC AACCIIDD
Formic acid is far more aggressive than acetic acid due to its high rate of
dissociation in water.
ENVIRONMENTS
MAIN
PROBLEM
Recommendation for a pure environment
Formic acid General surface
attack
* At ambient temperature
UGIMA®4307
* For concentrations < 1% or in 100% concentrated
acid at high temperature.
UGIMA®4307
* At a temperature < 80°C, whatever the
concentration:
UGIMA®4404
* At temperatures > 80°C and < 95°C, whatever the
concentration:
UGI4362
RRREEECCCOOOMMMMMMEEENNNDDDAAATTTIIIOOONNNSSS FFFOOORRR UUUSSSEEE IIINNN IIINNNDDDUUUSSSTTTRRRIIIAAALLL SSSEEEAAA WWWAAATTTEEERRR
DDDEEESSSAAALLLIIINNNAAATTTIIIOOONNN UUUNNNIIITTTSSS
There are two types of processes:
- Physical processes: RO (reverse osmosis) for large units,
- Thermal processes: MSF (multi stage flash) which represents 90% of the
market. The thermal process involves the risks of pitting and crevice
corrosion (if the dissolved oxygen content exceeds 1 ppm) and stress
corrosion cracking (SCC) if O2 > a few ppb.
Grade 4404 is not sufficient to withstand pitting and crevice corrosion in the event
of an increase in the percentage of oxygen; similarly, if brine residues have been
deposited on the walls, 316L will not be sufficient to withstand pitting and crevice
corrosion.
ENVIRONMENT
MAIN
PROBLEM
Basic
recommendations
AGGRAVATING
FACTORS
UGITECH
RECOMMEND-
ATIONS
Sea water
desalination
Pitting
corrosion
UGIMA®4404
Or
UGI 4362
Increase in the
percentage of
oxygen
UGI 4462
Multistage
process
Crevice
corrosion
UGIMA®4404
Or
UGI 4362
Brine deposits UGI 4462
Multi stage flash
(MSF)
Stress
corrosion
Temperature up
to 120°C in an
aerated
environment
UGI 4462
Formic acid
Sea water
desalination

- 29 -
3. Use of duplex steels in the building industry: for example concrete
reinforcing bars
This section deals with pitting corrosion in concrete reinforcing stainless steel bars.
OUR ACCELERATED CORROSION ELECTROCHEMICAL TEST CONDITIONS
(simulation test developed at the CRU in collaboration with the CEA)
 Determination of the pitting potential by electrochemical tests in environments
simulating the "concrete" solution in contact with the steel reinforcement: the more
positive the value obtained, the better the corrosion resistance.
 Consideration of the change in composition due to changes in this environment
over time: carbonation; the pH decreases from 12 to 8; presence of chlorides.
 The presence of chlorides is highly exaggerated in our experimental conditions:
the concrete would be so cracked that sea water would be able to penetrate
through to the steel reinforcement!
Figure 13: examples of the measurement of pitting potentials in a "concrete"
environment after 50 years of exposure in a marine environment

- 30 -
-600
-400
-200
0
200
400
600
800
1.4597 1.4301 1.4404 1.4462 acier 1.4362
pitting potential in mV/ ECS in a carbonate-bearing environment Na2CO3 + NaCl to 21 g/l
in chlorides and at pH=10
Figure 14: examples of the measurement of pitting potentials in a "concrete"
environment after 25 years of exposure in a marine environment
OOUURR RREECCOOMMMMEENNDDAATTIIOONNSS::
 Duplex steels are recommended for reinforcing bars, as their
higher mechanical properties allow the amount of reinforcement
required to be considerably reduced, leading to a consequent
reduction in building costs.
 UGIGRIP 4362 is better than 1.4404 in this environment and
under these test conditions.
 UGIGRIP 4462 is the most corrosion resistant and is
recommended for concrete structures in highly aggressive
environments such as: bridge piers in the sea, etc.
4. Use of duplex steels in the petrochemical industry: stress corrosion
problems.
The stress corrosion resistance of Duplex grades is excellent, due, on the one hand,
to their high mechanical properties and, on the other hand, to the fact that it is
difficult for cracks to propagate in a two-phase austenite–ferrite structure.
This type of corrosion is generated by acid chloride environments which often
contain hydrogen sulphide (H2S) pollutants; they are mainly encountered in the oil
industry and off-shore ("acid pits").
Duplex grades again appear to be far superior to austenitic grades in this field and
are particularly attractive in terms of cost.

- 31 -
It should be noted that in the long product sector, which mainly manufactures
mechanical components with varying degrees of stress, resistance to this type of
corrosion is more appreciated by users.
An H2S temperature versus pressure diagram showing the scope of use of the
various grades (to the left of the thick line) is illustrated below.
UGI 4462 and UGI 4507 are used far more widely than grade 4401.
Figure 15: corrosion resistance in an H2S "sour gas" environment
-30
20
70
120
170
220
0,01 0,1 1 10
Temperature in °C
Pressure in H2S
in bars
UGIMA® 4404
UGINE 4462
UGINE 4507
UGINE 4539

- 32 -
AAddddiittiioonnaall iinnffoorrmmaattiioonn
BIBLIO 1: Phosphoric acid - Natural phosphate attack stage:
The most widely used method is the wet process which involves attacking the
natural tricalcium phosphates Ca3(PO4)2 with a concentrated solution of sulphuric acid at
temperatures of between 80 and 110°C; the process is carried out when calcium fluoride
CaF2, silica SiO2, and CaCl2 and NaCl chloride impurities are present.
This stage produces a slurry consisting of:
- 30% P2O5
- polluted by fluorinated compounds in the form of HF (0 to 0.2%) and/or
hydrofluosilicic acid H2SiF6 (0 to 1.5%)
- polluted by chlorides (500 to 3000 ppm)
- containing solids non-reactive silica (quartz) and calcium sulphate.
Corrosion problems at this stage of the process with respect to the stirrers and pumps used
to transfer the slurry:
The entire surface may become highly corroded, due to depassivation of the materials in the
presence of fluorinated compounds, excessive amounts of H2SO4 and chlorides. In addition,
there are considerable abrasion phenomena.
BIBLIO 2: Phosphoric acid - Slurry filtration stage
This stage takes place at a temperature below 50°C and with no abrasion phenomenon. Its
purpose is to remove the calcium sulphate.
BIBLIO 3: Sulphuric acid manufacturing process and the corrosion
problems encountered:
The most widely used method is the "contact" process which uses vanadium pentoxide as a
catalyst to obtain concentrated acid.
Stainless steel is mainly used for hot concentrated acid transfer lines (60 to 110°C), drying
and absorption columns and acid coolers; the converters are therefore made of stainless
steel, sometimes with in-built gas/gas exchangers.
The main corrosion problem is a uniform or general attack over the entire surface. A pitting
corrosion phenomenon may be encountered in acids containing chloride impurities.
BIBLIO 4: Sulphuric acid with oxidising impurities
Such oxidising impurities can have a beneficial or an adverse effect on corrosion. In fact, if
the steel is in the active domain (which may be the case even if the corrosion rates are low,
for example at low temperature) the presence of an oxidising agent increases the dissolution
rate. The action of the oxidising agent is therefore beneficial if (and only if) the material is
in, or is brought into the passive domain.
The presence of reducible compounds in the solution results in the passivation of corrosion
resistant materials.
Example: in the zinc hydrometallurgical industry, certain stages in the process produce
sulphuric solutions with a concentration of 10 or 20%, at temperatures in the region of
100°C. Under such conditions, no grade can have the correct resistance; UGI 4539, UGI

- 33 -
4507 or UGI 4462 can nevertheless be used, as they contain traces of ferrous and ferric
ions.
BIBLIO 5 / Sodium carbonate or pure potash:
For concentrations less than 30%, UGIMA® 4307/304L is appropriate in pure soda or
potash; for example at 30% (or 20%) a temperature of 120°C (or 150°C) is possible.
However, for concentrations greater than 30%, the use of UGI 4539 / 904L and duplex
grades would appear mandatory if the temperature exceeds 90°C.
BIBLIO 6 / RECOMMENDATIONS FOR USE IN AMMONIUM CARBAMATE
AND UREA
The first stage in the manufacture of urea is to synthesise the ammonium carbamate by
reaction between CO2 and NH3 at high pressure. In the second stage, the carbamate is
converted into urea by dehydration at high temperature (150 to 200°C).
The liquid ammonium carbamate and the urea aqueous solutions are very aggressive, all the
more so as the temperature is always very high.
Summary of the effects of the different components:
Chromium Very beneficial for corrosion resistance. Optimum between 19 and 25%
Molybdenum Beneficial. Aim at a content > 2.5%.
Nickel
Adverse effect in this environment.
However, it prevents localised dechromisations following the sigma phase
formation due to traces of ferrite during instrument welding operations.
Copper Uncertain effect Never adverse for a content < 2%
Manganese Not decisive. Not adverse if Cr>19% and Mo>2.5% and Ni<6%.
Nitrogen < 0.2%. Prevents the precipitation of harmful intermetallic phases.
The main components are molybdenum and more particularly chromium, which explains why
a type 1.4592 grade is economically optimum.
However, this type of grade is not necessarily easily available in any product.
There are two alternatives:
 A completely austenitic grade, type 25-22-2 / 1.4466, with a high chromium
content
 Or a superduplex.
Superduplex poses a problem with respect to pressure vessels (in this case reactors) where
stress must be relieved at the end of the manufacturing process, owing to the demixing of
the ferrite in the sigma phase and chromium carbides at the conventional temperatures used
(550-580°C). That is why 25-22-2 / 1.4466 is recommended in this case.
Our recommendation: UGI 4507
Ferritic grades with a high chromium content (20 to 25%) and a high
molybdenum content (2.5%) such as type 4592 may possibly be used.
Urea

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